California Oakworm

The coast live oak is an important part of both the wild and urban landscape through most of coastal California. When these beloved trees are defoliated by our native oakworm it can be a dramatic sight for the home owners.  A healthy oak can withstand defoliation, but unhealthy trees can lose limbs or become susceptible to other more serious issues under the added stress from this caterpillar attack. It’s important to speak with your Arborist to determine the health of your trees, especially if they are near structures or commonly used outdoor areas like picnic tables or play sets.

The frequency of infestation is highly irregular, but we often find that after a heavy outbreak the tree will be free of caterpillars for several years. It’s important to remember that the oakworm is a native pest and plays a role in the local ecosystem by feeding a host of birds and insects. The role of oakworm in our environment should be considered before treatment is performed on any individual trees. Sometime we can treat important trees over patio and outdoor recreation areas and leave more remote trees to follow the normal pattern of defoliation and regrowth. This helps the homewner enjoy their outdoor spaces without significant impact on the local ecosystem.

Identifying the problem is the first step to any action regarding pests in trees and our oaks do have other issues that look similar. Pumpkin gall and even defoliation from root rot are commonly misdiagnosed by homeowners as an oakworm attack and the consequences can be dramatically different. A correct diagnosis is easily obtained from a Certified Arborist. The caterpillars can reach three centimeters in length with yellow stripes down their backs. The head is dark brown to red in color. The moths are brown to gray and can be found by shaking a low limb while they are active; usually you see them at dusk and dawn fluttering below and within the canopy. While young, they feed on the soft underside of the leaves near the base and expand to skeletonize the entire leaf as they become able to digest the thicker leaf material.

California Oakworm Caterpillar and Moth

While this pest is rarely fatal, it can become an aesthetic issue when it defoliates specimen trees in the landscape or when the caterpillars congregate on outdoor surfaces like walls and patio furniture. There are treatment options, but because of the irregular frequency of infestation and the multiple life-cycles each year it’s important to understand the pest to achieve good control. When you know you don’t want caterpillars in a specific tree we have a systemic injection that can protect a tree for a full year. While these injections work well, they do take time to spread throughout the tree so they are used primarily as a preventative approach for important trees. Once an outbreak has begun, spraying is the best option. We have a range of products that work depending on the stage of the caterpillars.

Whenever you suspect a pest or structural problem with a tree its best to contact your Arborist to make sure it’s dealt with properly and in a timely manner.

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